Volumetric imaging or volume rendering provides a graphical representation of a measured three-dimensional object or subject and can be used to visualize the internal structure of the object or subject. For example, in medical imaging volumetric scanners such as Computed Axial Tomography (CAT) scanners, Magnetic Resonance Imaging (MRI) scanners, Conebeam Computed Tomography (CT) scanners, or other types of scanners can be used to produce volumetric data of a scanned object such as part of a patient's body. From this volumetric data, a volumetric image of the scanned object can be produced. The volumetric image might be a three-dimensional object representation using volumetric rendering technology such as ray tracing or surface rendering after converting the volumetric data into an iso-surface model. As used herein, “subject” is used in context to refer to that physically present object, patient, patient body part, thing, etc. that is observed, scanned, etc. by an input device such as a scanner.
Images rendered based on the volumetric data of a subject can be adjusted to present different views of the subject. For example, by changing the threshold of opacity and transparency control, the internal shape or external shape and/or structure of the subject can be visualized. In medical applications, the image threshold can be adjusted to visualize the hard tissue or soft tissue, i.e., bones or muscles and flesh. As the volumetric image is adjusted to visualize the external shape of the subject, the outcome can be very similar to that provided by a three-dimensional surface scanner. That is, when there is sufficient contrast between the external volume density and the surrounding air or other containing background material, the surface of the subject can be very clearly visualized. For example, in medical applications, the three-dimensional facial profile of the patient can be clearly created and visualized.
Even though the goal of the volumetric scan of a subject is not necessarily to visualize the external profile of a subject, the external profile may be helpful if it is shown along with the internal structure. For example, in medical applications like plastic surgery, orthodontics, oral and maxillofacial surgery, the facial profile along with the underlying skull structure can provide useful information for diagnosis or development of a treatment plan.
While the three-dimensional external profiles created in such a manner can produce precise geometric representations of the shape of the surface of the subject, the color or the texture of the subject's external surface is missing. For some applications, an optical view of the external profile is very useful. For example, in plastic surgery or orthodontics applications, the optical view of the face can produce better definition of the eyes, lips, and eyebrows. Together, the three-dimensional shape and the texture can create a virtual patient model. Such images can enhance the diagnosis, treatment plan, or patient presentation.
Some three-dimensional optical or laser surface scanners can produce three-dimensional models with the color texture, but volumetric scanners cannot reproduce the surface texture by their nature since volumetric scanners detect physical properties of the mass that makes up the subject, not the optical reflection of the surface of the subject.
In order to obtain a the surface texture and the volumetric image of the subject, a multimodal representation can be used. That is, the volumetric image produced by a volumetric scanner and the textured surface scan model produced by a surface scanner can be put together into merged data. For example, the merge of face surface optical scans and the volumetric image can produce a virtual face model that can show a photorealistic face with an internal skull structure superimposed. Such a merged model can be very effective and useful for plastic surgical applications and orthodontic applications.
One of the challenges for such multimodal image representation is the registration problem. Since two sets of data, the surface scan data and volumetric scan data, are created independently, they are in two different coordinate systems. In order to superimpose the surface scan data on the volumetric scan data accurately to produce an accurate view, the two sets of data must be registered in the same coordinates in three-dimensional space. Such image and model registration may require extra steps, such as setting up common reference datum features before scanning or producing a computational best-fit of the data after scanning. Even with the extra steps, there are sources of errors and a consistency problem. Thus, the multimodal registration from two independent scans could be time consuming and unreliable.
Integrated scanner systems have been used to resolve the multimodal registration problem. An integrated scanner system comprises a surface scanning unit, such as a laser scanner, and a volume scanning unit, such as a CT scanner, in a common structure, i.e., the sensors are mounted in the same housing. Such an integrated scanner can create both the surface scan data and the volumetric scan data at the same time. Since the surface scanner and the volumetric scanner are physically mounted in the same housing, the surface scan data and volume scan data can be consistently aligned in the same coordinate system. In such a case, there is no need for the extra steps of pre-scan datum reference setup or post-scan best fit matching.
However, such an integrated scanner system might not be commercially viable. First, the cost of the system increases since the system requires both a volume scanner and a surface scanner. Second, the size of the system could increase as the two units have different spatial requirements. Third, memory and computation demand more resources since the data comprises of volumetric scan data, surface scan data and color texture data.
Hence, there is a need for improved methods and systems for generating a volumetric image with a color textured external surface that do not require the additional expense, size, and computational resources of a surface scanner unit, do not present a need for registration of the images, and can be adapted to existing volume scanners.